1. Make the dough: Cut the butter into ½-inch cubes and place in freezer while assembling the remaining ingredients.

2. In a food processor fitted with the steel blade, place the flour, sugar, and salt. Pulse a few times to combine. Add the cold butter. Pulse 15 times, or until the butter particles are the size of small peas. With the motor running, add ¼ cup ice water through the feed tube. Process for about 10 seconds, stopping the machine before the dough becomes a solid mass (add more water, if needed). Turn the contents of the bowl onto a piece of plastic wrap, pressing any loose particles into the mass of dough. Roughly form the dough into a disk. Cover the dough completely with plastic wrap and refrigerate for at least 1 hour (or freeze for 15 minutes). (May be refrigerated up to 2 days or frozen for up to 2 weeks.)

3. Preheat the oven to 450 degrees.

4. Place the dough on a sheet of parchment. Cover with a piece of plastic wrap and roll to a 12-inch free-form circle. Transfer with the parchment to a baking sheet. Remove the plastic wrap and sprinkle with 1 tablespoon of the sugar.

5. Leaving a 1½-inch border all around, cover the dough with the raspberries and blueberries, stem ends down. Starting in the center, work your way toward the outside in concentric circles. Most of the berries will fit on the shell in one layer; mound any extra berries in the center. Sprinkle the berries with the remaining sugar. Raise the dough border to enclose the sides of the tart, letting it drape gently over the fruit. Gently pinch the soft pleats that form the draping to secure the edges.

6. Bake for 20 to 25 minutes, until the fruit has given off some of its juice and the dough is golden. Cool on a rack for about 10 minutes. Cut the tart into 8 slices. Dust with powdered sugar.